Fast user switching (accessible from Login Options in the Accounts System Preference pane) makes it easy to keep two or more users logged in to a Mac at the same time. Instead of logging in and out when you need to switch accounts, you can flip between them with a quick icon click in the menu bar. This is huge timesaver for people who need to use more than one account—family members, people giving presentations, users trying to debug their main account, and even writers who take screenshots for articles and blogs. But since the release of Mac OS X 10.4, fast user switching has also suffered from a small, but annoying, bug.

This bug only manifests itself if you use a custom color profile. That may sound like some really fancy feature that only high-end designers would use, but if you’ve used the Color tab of the Displays System Preferences panel to calibrate your monitor at all, then you’re using a custom color profile. I hope you’ve done this if you do much work with your own photos (see how
here
). There’s also a good chance you’ve done this if you simply find the default OS X color profile too bright or washed out looking.

Here’s what happens: Say you switch from your Main account to your Test account. So far, so good—you won’t see any change in your display. But when you switch back to your normal account, you suddenly find yourself staring at your Mac’s default color scheme. The change is something that’s hard to demonstrate well, but here’s an image that tries:

To my eye, the left side of the image (which shows the generic color profile) looks quite washed out compared to the right (my custom profile)—the green background is much lighter, and the shadow around the window is notably lighter. Even the brushed metal in the window’s title area looks too light. That’s why I tweaked the default color set-up in the first place. (If you’d like to see a full-size, higher-quality JPEG, just click the above image.)
When you try to reactivate the customized color profile by taking a quick trip to the Color tab in Displays, OS X just beeps at you. Clicking Calibrate doesn’t work either; you just see an error message. So much for the simple solutions. I (along with many others) had just put up with a washed-out screen when I used fast user switching, as that was better than what was required to get everything looking proper again—rebooting the Mac.

If you’re feeling puzzled at this point because you
haven’t
experienced this bug, note that it doesn’t seem to affect everyone. Fast user switching works without a hitch on my PowerBook, but I run into the bug on my desktop Mac. Others have reported no issues at all. If you’re not affected, consider yourself lucky; if you are, keep reading for the solution.
Lucky for us, an intrepid
macosxhints.com
reader dived into (or,
depending on where you live, you may prefer to think the reader dove into…) the depths of OS X to figure out what was going on. After much exploration, he came out with the solution. The guilty party is a tiny little application named DMProxy, buried very deeply in the system. To find DMProxy, by far the easiest thing you can do is copy and paste the following command into Terminal (in Applications/Utilities). Launch Terminal, then paste the following text:

open /System/Library/Frameworks/ApplicationServices.framework\
/Versions/A/Frameworks/CoreGraphics.framework/Versions/A/Resources/

After pasting, just press Return. When you do, the Finder will open a new window showing the folder you specified in that long, complex path. Inside that Resources folder, you’ll see the DMProxy application icon:

Now hold down Command and Option, then drag the DMProxy icon to a convenient location, such as the Applications folder or your Documents folder. This creates an alias to the original file, so you won’t have to use Terminal to find it in the future.

Now, whenever you switch back from another account and experience the bug, just launch the DMProxy application. Apple’s Developer site doesn’t have any info on what DMProxy does, but it’s somehow involved in the graphics layer of OS X, given its location.

As soon as you do, your Mac will revert to your calibrated profile. Do this as necessary on any account that loses its color scheme. (It might make the most sense to keep the alias in Applications folder, so all users can get to it.) In my testing, I only had to run this application once when the problem occurred, and I could then fast user switch without the problem recurring—at least until the next time restarted my Mac.

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